When we brought home a third cat, we had her in a cage or good sized dog crate, in the middle of the living room. The first two got to know her but were unable to get near enough to have a conflict and the third cat was able to see how everyone interacted. After a few days, they were all used to seeing each other nad we had no problems. However, Himi's can be a bit difficult. Superiority complex, I think.